Implementation of delay-critical services in a cable television system

ABSTRACT

For transmitting delay-critical data in a digital, time division form in an additional channel of a cable television system, the slots of the channel are further divided into smaller mini slots, for the indication of which the same cyclic indication is used as with which the superframes controlling the use of the channel indicate the original slots. A method based on mini slots is compatible with DAVIC 1.0 and 1.1 specifications.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to an arrangement of data connectionsassociated with communication services such as telephone, meeting andgame services which require as short a data transmission delay aspossible.

2. Discussion of Related Art

A cable television system is most conventionally a distribution networkresembling a tree, at the root of which lies the main amplifier of theoperator controlling the network, i.e. the so-called head-end. The mainamplifier may more generally be called a central configuration. Thedistribution lines branch from it in a tree-like fashion towards thedata terminal equipment of the subscribers, of which there may behundreds of thousands under one central configuration. In order tocompensate the weakening of the signal and to reduce interferences, thedistribution lines comprise distributing amplifiers, repeaters and otherdevices known per se.

Lately, plans have been proposed to change the cable television systemsfrom one-way distribution networks to two-way data transmissionnetworks. In this case, the direction of data transmission from thecentral configuration to the data terminal equipment is generally calleddownstream (DS), and the reverse direction upstream (US). In addition toa high-capacity downstream main channel, the system includes additionalchannels; these include at least an upstream channel, through which thedata terminal equipment may transfer data to the direction of thecentral configuration, and a relatively low-capacity downstream controlchannel, though which the central configuration controls the use ofupstream connections. The control channel may consist of cyclicallyrecurrent fields which the central configuration multiplexes with thedigital video picture or another signal transmitted in the main channelby utilizing its frame structure. Such a control channel is a so-calledin-band control channel. In another embodiment, the control channel issituated on an own frequency band, thus being a so-called out-of-bandcontrol channel.

The present invention may be applied to digital video systems known inthemselves, i.e. DAVIC (Digital Audio Visual Council) and DVB (DigitalVideo Broadcasting). The system specifications significant for theinvention are found in the publications "DAVIC 1.0 specification part08; Lower layer protocols and physical interfaces, December 1995","DAVIC 1.0 corrigenda part 0.8; Lower layer protocols and physicalinterfaces, Edited version after New York meeting, Rev. 2.1, June 1996","DAVIC 1.1 specification part 08; Lower layer protocols and physicalinterfaces Rev. 3.3", and "ETSI draft specification prETS 300 800;Digital Video Broadcasting (DVB); DVB interaction channel for cable TVdistribution system (CATV), TM 1640 Rev. 4, June 1996". The cable TVsystem disclosed in the publications may be based on coaxial cables, orat least partly on optical fibres; in the latter case, it is also calleda HFC (Hybrid Fibre Coax) network.

FIG. 1 illustrates a proposition included in the publication prETS 300800 for allocating frequencies in a DVB system. The dimensions on thehorizontal frequency axis are indicative, and the vertical axis onlyshows which signals are directed towards the data terminal equipmentfrom the central configuration (DS, upwards in the figure), and whichextend to the reverse direction (US, downwards in the figure). Thefrequency range 100 extends from about 50 MHz to almost 900 MHZ, and itis typically divided into channels 102 of 6-8 MHz, of which only threeare shown for clarity. Each channel contains one QAM (QuadratureAmplitude Modulation) modulated signal which may include, for example,one or more digital video signals in a MPEG-TS format (Motion PictureExperts Group--Transport Stream) or other data requiring a high transfercapacity. The frequency range 103 extends from 70 MHz to 130 MHz, and itcontains channels 104, which are 1 or 2 MHz wide, each transferring aQPSK (Quadrature Phase Shift Keying) modulated control channel. Forclarity, of these only three are shown in the figure. The frequencyrange 105 extends from 300 MHz to 862 MHz, and its contents correspondto the frequency range 103. The frequency range 106 of the figure,extending from 5 MHz to 65 MHz, is reserved for upstream connections,and it contains channels 107, of which only three are shown in thefigure, and which are 200 KHz, 1 MHz or 2 MHz wide. The QPSK modulationis also intended to be used in upstream connections. The newspecifications of the DAVIC system are, however, also prepared to usethe QAM modulation in channels of low capacity.

A transmission especially in a channel of the DAVIC system containingdata about the use of one or more upstream channels, consists inaccordance with FIG. 2 of SL-ESF frames 108 (Signalling Link ExtendedSuperframe). The length of one SL-ESF is 4632 bits, and it is dividedinto 24 frames of 193 bits. In FIG. 2, the frames are numbered from 1 to24, and additionally, an enlarged frame is described with the referencenumber 109. Each frame starts with a so-called overhead bit 110, whichis followed by a payload 111 of 192 bits. The significance of theoverload bit depends on which frame of the SL-ESF is observed. In flames4, 8, 12, 16, 20 and 24 included in the SL-ESF, the value of theoverhead bit is a fixed framing bit, i.e. a so-called F bit.Correspondingly, the overhead bits of the frames 2, 6, 10, 14, 18 and 22are so-called C bits, which, when placed successively, form a CRCchecksum describing the bit contents of the previous SL-ESF. In everyother frame, beginning from frame 1, the overhead bit is an M bit, i.e.part of a so-called M counter indicating the numbering and timing of theslots of an upstream channel controlled in this control channel.

Communications in each upstream channel is divided into slots 112, asdescribed in FIG. 3. The central configuration determines the use of theslots so that part of the slots may be used for ranging aiming at themeasurement and compensation of transfer delays, part are conventionallyfreely available for data terminal equipment (so-called contention-basedslots), part has been determined for the use of data terminal equipmenthaving made a reservation in accordance with a specific reserveinventory (so-called reservation slots), and part has an operation timeschedule distributing a certain regular data transmission capacity forthe use of one connection (so-called contentionless-based slots). In theDAVIC system, the central configuration transmits data concerning theuse of eight upstream channels in one downstream channel.

The communication of downstream and upstream channels is synchronized sothat each downstream SL-ESF, the M bits M1, M5 and M9 (in SL-ESF, thesequence numbers of these bits from the beginning of SL-ESF are 0, 1544and 3088) correspond to the so-called slot position references. If thebit rate of a downstream channel is 1,544 Mbit/s, the period of twosuccessive slot position references accommodate three upstream slots,i.e. the temporal length of one downstream SL-ESF is the same as theadded temporal length of nine upstream slots. If the bit rate of anupstream channel is 256 kbit/s, one upstream slot is temporally as longas the time from one slot position reference over the next to the nextone so that the temporal length of one downstream SL-ESF is the same asthe added temporal length of one and a half upstream slots. If the bitrate of an upstream channel is 3,088 Mbit/s, the period between two slotposition references accommodates six upstream slots, i.e. the temporallength of one downstream SL-ESF is the same as the added temporal lengthof 18 upstream slots.

For the identification of downstream SF-ESFs and upstream slots, theyare numbered cyclically. The numbering of SL-ESFs runs from 9 to N,where N is the size of the cycle, i.e. the largest sequence number usedfor SL-ESF. The cycles mean that the SL-ESF number 0 is followed bySL-ESF number 1, which is followed by SL-ESF number 2, and so on, untilSL-ESF number N is again followed by SL-ESF number 0, and the numberingstarts all over. If the bit rate of a downstream channel is 3,088 Mbps,two sequential SL-ESFs are always given the same sequence number, i.e.the sequence number changes only at the place of every other SL-ESF. TheM bits M10-M1 of each SL-ESF form a 10-bit register where M10 is themost significant bit and M1 the least significant, and the value ofwhich indicates the sequence number of the SL-ESF. As the registerincludes 10 bits, the largest possible value for the number N is 2¹⁰.The data terminal equipment maintain the numbering of upstream slots,which is synchronized with the numbering of SL-ESFs. If, for example,the bit rate of 1,544 Mbps is used both in downstream and upstream, theslots from 0 to 8 correspond to the SL-ESF number 0 and the slots from 9to 17 correspond to the SL-ESF number 1, and so forth.

A data terminal equipment, to which a certain slot of an upstreamchannel has been assigned, transmits a burst of 64 bytes during the slotin question. A burst 113 is shown in more detail in FIG. 3. At thebeginning of the burst there is a synchronization period 114 of fourbytes, which is called Unique Word and which corresponds to the hexanumber sequence CC CC CC 0D. It is followed by a payload 115 of 53bytes, which most commonly is an ATM cell, and a six-byte Reed-Solomoncode 116 is calculated from its contents. At the end of the burst thereis a guard period 117 of one byte.

The upstream slots are grouped as shown in FIG. 3, the groups including3, 9 or 18 slots depending on the bit rate (256 kbps, 1,544 Mbps or3,088 Mbps). In the case of FIG. 3, each group contains 9 slots, i.e. itis an upstream channel of 1,544 Mbps. The data terminal equipment forwhich a certain regular data transmission capacity is assigned inaccordance with the contentionless principle, may, for example, receiveone slot from each group. In the case of FIG. 3, a certain data terminalequipment, which is denoted with A, has received the fourth slot inevery group. The data terminal equipment B makes it with half a smallerdata transmission capacity so that it receives one slot (in the Fig. thesixth slot) from every other group. The data transmission need of thedata equipment terminal C is further half of the data transmission needof B so that it receives one slot from every fourth group.

If the data terminal equipment requires more data transmission capacitythan the data terminal equipment A, it is assigned several slots fromeach group. In accordance with the cyclical indication of slots definedin the DAVIC system, the slots received by a certain data terminalequipment are defined so that they are situated equidistantly in astring of upstream slots. In order for this precondition to be inharmony with the fact that, for facilitating the indicationarrangements, the slots assigned for a certain data terminal equipmenthave to be situated in successive slot groups at same places in relationto the group's start and end, only certain multifolds of slots arepossible for achieving a larger data transmission capacity. Depending onthe bit rate of the upstream channel, the permitted multifolds are 1 and3 (with bit rate of 256 kbps); 1, 3 and 9 (with the rate of 1,544 Mbps),or 1, 3, 9 and 18 (with the rate of 3,088 Mbps). The assignment ofcontentionless-based slots is realized so that the Connect-messagetransmitted to the data terminal equipment by the central configurationincludes a so-called Cyclic₋₋ Assignment record of six bytes in length,which contains three fields of two bytes long, i.e. a start field, adistance field, and an end field. The central configuration uses thesefields to inform in which slot the data terminal equipment may start itstransmission, what is the mutual distance between the slots assigned toit, and in which slot the data terminal equipment has to end thetransmission.

The data transmission rate equalled by one slot in each slot group maybe called the root bit rate (RBR). When the bit rate of the upstreamchannel is 256 kbps, the root bit rate is 64 kbps. If the bit rate ofthe upstream channel is 1,544 Mbps or 3,088 Mbps, the root bit rate is128 kbps. The possible data transmission rates corresponding todifferent slot arrangements permitted for one data terminal equipmentare, for example, 1,152 Mbps, 384 kbps and 128 kbps in an upstreamchannel of 1,544 Mbps, and bit rates smaller than those above so thatthe next bit rate is always half the former.

When trying to apply the said state-of-the-art arrangement for arranginga two-way data transmission in a cable television system to carry outdelay-critical services, one is confronted with certain problems. Here,the delay-critical services refer to so-called communicative services inwhich messages in the nature of questions and answers are exchangedbetween the sender and the receiver, the questioner wishing to receivethe answer as soon as possible. The most common delay-critical servicesare telephone connections and various teleconference services, but alsosome multi-player games set similar demands for the shortness of delays.While the tele services are diversifying, new services are generated allthe time, part of which will always be delay-critical. The delay causesboth the slowing down of the communication and the disturbing echoing ofthe sent data from the receiver back to the sender.

A digital telephone connection in accordance with the publication ITU-TRecommendation G.711 is viewed as an example; this connection conveysdigital speech signals generated on a sampling frequency of 8 kHz andwith an 8-bit A/D conversion so that the bit rate required for datatransmission is 64 kbps. It has to be noted that data to be transferredis also generated with the same bit rate; if ATM cells with a payload of48 bytes (384 bits) are used for data transmission, filling an ATM cellwith the rate of 64 kbps takes 6 milliseconds. In order to optimally usethe capacity of the channel, the equipment does not transmit the ATMcell forward before it is full, so that merely filling the ATM cellcauses a delay of 6 ms to the signal. In the publication "Lee, Kyoo J.:Signal Delay Requirement for IEEE 802.14 protocol to support voiceapplication, IEEE 802.14, Jan. 10, 1996, IEEE 802.14-96/011" it isproposed that the reciprocating delay for the telephone service shouldbe less than 3,4 ms so it is obvious that the data transmission systembased on ATM cells described above is not able to provide delay-criticalservices in a desired way.

SUMMARY OF INVENTION

The object of the present invention is to provide a data transmissionsystem and equipment for the production of delay-critical services in acable television system, especially in a DAVIC system.

The objects of the invention are achieved by dividing the slots of theDAVIC system into shorter mini slots for the delay-critical service, andto arrange their assignment in a way compatible with the present DAVICsystem.

It is characteristic of a method of the present invention fortransmitting data in an additional channel of a cable television networkapplying a time division multiple access operation, in which time slotsare assigned for different equipment for the distribution of datatransmission capacity, and in which the operation of the slots in acable television network is controlled by operating assignmentstransmitted downstream, that the slots are further divided into minislots, the use of which is controlled by operating assignmentstransmitted downstream.

The present invention further relates to a cable television system,which is characterized in that the central configuration comprises meansfor regrouping the control commands so that they assign mini slots forthe single data terminal equipment, which are temporally shorter thanthe state-of-the-art slots, and that at least one data terminalequipment comprises means for transmitting data as mini bursts the sizeof a mini slot so that the data terminal equipment is provided totransmit data as bursts or mini bursts in accordance with the controlcommands it receives from the central configuration.

In the arrangement of the invention, the smallest separate unit of thedata transmission capacity given to the use of one data terminalequipment is a mini slot, the size of which is selected mostadvantageously so that the added temporal length of three successivemini slots is substantially the same as the length of onestate-of-the-art slot. It is also possible to determine a mini slot ofanother size. For the numbering of mini slots, an expansion of cyclicalnumbering of the state of the art is used, and their operation iscontrolled by control messages included in the downstream controlchannel SL-ESF. The mini slot arrangement of the present invention iscompatible with the DAVIC 1.0 and 1.1 specifications, but it providestwo-way cable television communications with a new data transmissionmode of the STM type (Synchronous Transfer Mode) applicable especiallyto the realization of delay-critical services. As the payload part of aburst transmitted within a mini slot has considerably fewer bits thanthe payload of an ATM cell, a device transmitting in accordance with theinvention does not cause a relatively long delay of the state of theart, which is described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is next described in more detail referring to anexemplary advantageous embodiment and to the enclosed drawings, inwhich:

FIG. 1 illustrates a known proposition for the distribution offrequencies in a DVB system;

FIG. 2 illustrates the distribution of transmission in a known controlchannel;

FIG. 3 illustrates the distribution of transmission in a known upstreamchannel;

FIG. 4 illustrates a slot structure of the invention;

FIG. 5 illustrates the synchronization arrangement of slots of theinvention; and

FIG. 6 illustrates the exchange of messages between the centralconfiguration and the data terminal equipment.

FIG. 7 illustrates a cable television system comprising a centralconfiguration and data terminal equipment where data is transmittedtherebetween over cables for implementation, according to the presentinvention, of delay-critical services.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In connection with the description relating to the state of the art,reference is made to FIGS. 1-3 so that the description of the presentinvention and its advantageous embodiments refers mainly to FIGS. 4-6.In the figures, the same reference numbers are used of correspondingparts.

In the description of FIGS. 4-6, which follows, reference may be had tothe cable television system illustrated in FIG. 7, which comprises acentral configuration and several data terminal equipment in a datatransmission connection, where data is transmitted by cables. Thecentral configuration comprises means for receiving data from dataterminal equipment in accordance with a time-division principle in slotsand includes means for establishing control commands to assign the slotsfor the use of separate data terminal equipment. In such a cabletelevision system, at least one data terminal equipment includes meansfor receiving control commands from the central configuration andfurther includes means for transmitting digital data to the centralconfiguration in burst form in a slot defined by the control commands.According to the present invention, the central configuration furthercomprises means for resynchronizing the control commands so that theyassign mini-slots for the use of separate data terminal equipment, whichare temporally shorter than the slots. Moreover, the data terminalequipment further comprises means for transmitting data as mini-burststhe size of a mini-slot so that the data terminal equipment is providedto transmit data as bursts or mini-bursts according to what kind ofcontrol commands it receives from the central configuration.

FIG. 4 illustrates a mini slot structure which is considered to be themost advantageous embodiment of the invention. Here, the added temporallength of three successive slots is substantially the same as the lengthof one upstream slot according to the state of the art. For comparison,FIG. 4 also illustrates a known slot 112 and a known burst 113. Atransmitting equipment according to the state of the art transmits aburst 113 in a slot 112, the burst consisting of a synchronization block114 of four bytes, a payload part 115 of 53 bytes, and a six-byteReed-Solomon code 116 calculated from its contents. At the end of theburst there is a guard period 117 of one byte. The device transmittingin a mini slot 118 of the invention transmits a burst 119, which may becalled a mini burst and at the beginning of which there is a similarsynchronization block 120 as at the beginning of a state-of-the-artburst. The payload part 121 of the mini burst is only 14 bytes (112bits) long. For error correction, the mini burst includes a two-byte(16-bit) CRC code 122, and the end of the mini slot includes a guardperiod 123 of one byte. The length of the mini burst in bytes, the guardperiod included, is 21 bytes so that the length of three successive minibursts is 63 bytes. After every third mini burst, the slot structure ofthe invention has an additional guard period 124 so that the length ofthe structure would be the same as the length of the state-of-the-artslot 112 (64 bytes).

In an upstream transmission, the mini slots are synchronized to theSL-ESFs of the control channel controlling the upstream channel in thesame way as the larger slots according to the state of the art. In FIG.5 it is assumed that the bit rate in both the control channel and theupstream channel is 1,544 Mbps. The line 125 illustrates successiveSL-ESFs of the downstream control channel. The line 126 illustrates thescheduling of the mini slots of the invention in relation to SL-ESFs,and the line 127 illustrates the corresponding scheduling of thestate-of-the art slots, for the sake of comparison. When the bit rate ofthe upstream channel is said 1,544 Mbps, exactly 27 successive minislots are scheduled for each downstream SL-ESF. The black diamonds belowthe line 125 illustrate the slot position references included in theSL-ESF in a known way. As there are three of these in each SL-ESF and asthey form an equidistant line, exactly nine mini slots are scheduledbetween two successive slot position references.

In the specifications of the DAVIC system mentioned earlier, it has beenshown how the central configuration determines which slots of theupstream channel are ranging, contention-based, reservation andcontentionless-based. The invention does not require any changes to bemade in this arrangement, as in accordance with an advantageousembodiment of the invention, the three mini slots which are placed "atthe place" of a certain slot according to the state of the art, are allof the same type. The control channel indicates the common type of thesaid mini slots in a similar way in which it would indicate the type ofa state-of-the art slot. In the DAVIC system, the ranging operationaiming at the measuring and compensation of transfer delays isdetermined so that the mini slots are not applicable to it; thus, themini slots are either contention-based, reservation, or contentionless.The mini slots of the contentionless type, mainly, are suitable forproviding telephone connections and other services requiring a constantdata transmission capacity.

The division of an upstream channel into slots or mini slots is calledgranularity. The central configuration states the granularity of acertain upstream channel, for example, by including the code numberrelated with it to a certain two-bit reservation control field, which isincluded in the so-called R-bytes of the SL-ESF transmitted in thecontrol channel. According to a proposition, the bit value "10" of thereservation control field (corresponding to the number 2 in the decimalsystem) indicates that the granularity of the upstream channel is of asize of a slot according to the state of the art, and the bit value "11"(corresponding to the number 3 in the decimal system) indicates that thegranularity of the upstream channel is the size of the mini slot. Alsoin the latter case, the upstream channel may also be used to transmitstate-of-the-art bursts, as long as the data terminal equipmenttransmitting them has three successive mini slots at its disposal. In anupstream channel, the granularity of which is the size of a mini slot,the possible ranging slots are state of the art, and the slots of thecontention type are always mini slots, because they are not reserved forany specific data terminal equipment. Slots of the reservation andcontentionless types may either be mini slots, or they may be reservedfor a specific data terminal equipment three in succession so that thedata terminal equipment may transmit state-of-the-art bursts.

The numbering of mini slots of the invention are next described inrelation to the numbering of the control channel SL-ESFs. The SL-ESFscontain slot position references in a known way. The number of minislots in an upstream channel between two successive slot positionreferences and the total number of mini slots in use in one seconddepend on the bit rate of the upstream channel in accordance with thefollowing table.

                  TABLE 1                                                         ______________________________________                                        Bit rate of an Mini slots/slot                                                                           Mini                                               upstream channel                                                                             position reference                                                                        slots/second                                       ______________________________________                                        256      kbps      1.5         1500                                           1.544    Mbps      9           9000                                           3.088    Mbps      18          18000                                          ______________________________________                                    

The changes required by the invention to the algorithm, which is shownin the specifications of the DAVIC system and which the data terminalequipment uses for maintaining the slot numbering, are limited to thedata terminal equipment giving the value read according to the bit ratefrom the middle column of the table 1 to the value of the number m. Thenumber m is a parameter present in the said algorithm, which in thestate-of-the-art version determines how many slots are accommodatedbetween two successive slot position references. In enclosure 1, thealgorithm is shown both in form of the specification and the changedform of the invention.

FIG. 6 illustrates the exchange of MAC (Media Access Control) messagesbetween the central configuration (NRC, Network Related Control) and thedata terminal equipment (NIU/STB, Network Interface Unit/Set Top Box)for establishing the connection. Messages 130 and 131 belong to theso-called Initialization and Provisioning phase, messages 132-136 formthe so-called Sign-On phase, and messages 137-139 the actual ConnectionEstablishment phase. If data concerning the granularity of the upstreamchannel is transmitted as previously proposed in the R bytes of theSL-ESFs of the control channel controlling the said channel and it isnot referred to in the contents of the message of FIG. 6, the presentinvention requires changes only in the Connection Establishment phaseformed by the messages 137-139, and here, mainly in the message 137(Connect message). Enclosure 2 illustrates in table form the connectmessage according to the state of the art and according to anadvantageous embodiment of the invention. The first change concerningthe Connection₋₋ Control₋₋ Field of the message, which in thestate-of-the-art message is one byte long. According to an advantageousembodiment of the invention, the field is extended to two bytes so thatit contains five bits reserved for later use and three new flag bits.The first flag bit indicates whether the connect message includes aso-called second Cyclic₋₋ Assignment record, to the contents of whichwill be reverted later. The second and third flag bits indicate, whetherthe connect message includes a downstream and upstream connectiondescription of the STM type, which is in the following called DS₋₋ STM₋₋CBD and US₋₋ STM₋₋ CBD, the abbreviation CBD being short for ConnectionBlock Descriptor, in accordance with the established practice in theDAVIC system.

Most advantageously, the second Cyclic₋₋ Assignment record is an 8-byterecord, which the central configuration may include in the connectmessage and which is shown in the part of enclosure 2 illustrating theinvention. It has four fields of two bytes, of which the start, distanceand end fields have the same meaning as in the first Cyclic₋₋ Assignmentrecord according to the state of the art. When required, the centralconfiguration uses the second start field (Contentionless₋₋ StartSecond) for giving the data terminal equipment a second starting slot,from which the data terminal equipment may continue the transmission atintervals indicated by the distance field. The second start field onlyhas to determine one starling slot; an easy way is to determine that ifthe value of the second start field in the second Cyclic₋₋ Assignmentrecord is the same as the value of the end (Contentionless₋₋ End) field,the definition of the second starting slot is not valid. The Connectmessage may include either the first Cyclic₋₋ Assignment record, only,the second Cyclic₋₋ Assignment record, only, or both the Cyclic₋₋Assignment records so that in the latest alternative each slot group mayinclude three slots determined specifically for the data terminalequipment and their permitted recurrence cycles. Depending on whichrecords are included in the Connect message, the central configurationmay give one data terminal equipment the use of the following mini slotmultifolds for a slot group when it is required that the position of theslot or slots assigned for the data terminal equipment in relation tothe stall and end of the group remains the same from one group toanother.

                  TABLE 2                                                         ______________________________________                                        Bit rate                                                                              Number                                                                of an   of mini  Permitted Permitted                                                                              Permitted                                 upstream                                                                              slots in multifolds,                                                                             multifolds,                                                                            multifolds,                               channel slot group                                                                             first record                                                                            second record                                                                          both records                              ______________________________________                                          256 kbps                                                                             9       1, 3, 9   1, 2, 3, 6, 9                                                                          1, 2, 3, 4, 5, 6,                                                             7, 9                                      1,544 Mbps                                                                            27       1, 3, 9, 27                                                                             1, 2, 3, 6, 9,                                                                         1, 2, 3, 4, 5, 6,                                                    18, 27   7, 9, 10, 11,                                                                 12, 15, 18, 19,                                                               21, 27                                    3,088 Mbps                                                                            54       1, 2, 3, 6, 9,                                                                          1, 2, 3, 4, 6, 9,                                                                      1, 2, 3, 4, 5, 6,                                          18, 27, 54                                                                              12, 18, 27, 36,                                                                        7, 8, 9, 10, 11,                                                     54       12, 13, 14, 15,                                                               18, 19, 20, 21,                                                               22, 24, 27, 28,                                                               29, 30, 31, 33,                                                               36, 37, 38, 39,                                                               42, 45, 54                                ______________________________________                                    

The basic bit rate of an upstream channel divided into mini slots is 16kbps in a channel of 256 kbps, and 32 kbps in channels of 1,544 Mbps and3,088 Mbps. Concerning the permitted multifolds, it especially has to betaken into account that in a channel of 256 kbps, the quadruple bit ratein relation to the basic bit rate, and, in other channels,correspondingly, the double bit rate in relation to the basic bit rate,is 64 kbps, which corresponds to the data transmission need of theafore-mentioned digital telephone system.

Connection descriptors (CBDs) are records which the centralconfiguration uses for transmitting the identifier, frequency and typeof a specific connection to the data terminal equipment. Thespecifications of the DAVIC system determine the connection descriptorsfor a downstream ATM connection, a downstream MPEG program transmission,and an upstream ATM connection. Because the present inventionestablished a connection of the STM type between the centralconfiguration and the data terminal equipment, which may bebi-directional, it is proposed to determine two new connectiondescriptors. The new connection descriptors are presented in table formin enclosure 3. According to an advantageous embodiment of theinvention, the connection descriptor of a downstream STM connection is arecord of seven bytes in length, in which the four first bytes indicatethe frequency of the downstream STM type connection in the same way asin known connection descriptors. The next two bytes indicate theunequivocal identifier of the downstream STM connection, which inenclosure 3 has been marked Downstream₋₋ STM₋₋ ID. The last byte is acode number indicating the modulation method used in the connection inthe same way as in state-of-the-art connection descriptors.

According to an advantageous embodiment of the invention, the connectiondescriptor of an upstream connection of the STM type is also a record ofseven bytes in length, in which the four first bytes indicate thefrequency of the upstream STM-type connection in the same way as inknown connection descriptors. The two next bytes indicate theunequivocal identifier of the upstream STM-type connection, which in theenclosure 3 is denoted with the name Upstream₋₋ STM₋₋ ID. The last byteis a field containing similar data as the Upstream₋₋ Parameters field ofthe connection descriptor concerning a known upstream ATM connection.

So that the data terminal equipment may be certain that the burst sentby it has reached its destination, the central configuration has totransmit some kind of an acknowledgement for each burst. In astate-of-the-art arrangement, the acknowledgement is included in form ofspecific indicator bits in the so-called R bytes in the SL-ESFs of thecontrol channel. If in the system of the invention the number relationsof the control channels and the upstream channels controlled by them andthe bit rates of the channels are determined so that one control channelis able to transmit a sufficient number of R bytes in relation to theupstream channels controlled by it, the same arrangement may also beused for acknowledging bursts transmitted in mini slots. However, inanother embodiment of the invention, the indicator bits included in theR bytes are only used for acknowledging the state-of-the-art bursts (ATMcells) transmitted in an area of three successive mini slots. For theacknowledgement of the mini bursts, it is in this case most advantageousto generate a new MAC (Media Access Control) message, which is in thefollowing called a Mini Slot Reception

Indicator Message and which is comparable to the MAC messages presentedin the DAVIC specification. In FIG. 4, there is presented a propositionfor a mini slot reception indicator message in table form. The length ofthe information parts of the message is 5 bytes, of which the two firstbytes contain the number of mini slots in which the first mini burst tobe acknowledged has been transmitted. The next byte indicates how manybursts starting from the first one are acknowledged by the samereception indicator message. The two last bytes contain theidentification number of the connection to which the bursts to beacknowledged are related.

Mini slots are well suited for the transmission of reservation messageswhich the data terminal equipment use for informing the centralconfiguration that they wish to reserve (mini) slots. A state-of-the-artreservation message is only 11 bytes long, so it is fitted as part ofthe 14-byte payload of a mini burst. Three bytes may be reserved forlater extensions of the system.

Above, the use of mini slots has been described especially for realizingdata transmission in an upstream channel. However, it is also possibleto apply the same principle to downstream channels using communicationrelated to separate data terminal connections divided into slots.Although it has been presented above that the basis for a delay-criticalservice is expressly a digital telephone signal generated with a speedof 64 kbps, the invention does not in any way limit the way how thesignal to be transmitted is generated. For one skilled in the art it isobvious that specifications of known cable television systems may bemodified further without departing from the inventive idea described inthis patent application and defined more closely in the appended claims.It is, for example, possible to increase the numbering space used in MACmessages from the present 2¹³ to 2¹⁶ ; thus, the size of the numberingspace would not limit the indication of the mini slots.

Compared with state-of-the-art solutions, the invention does not requireany significant changes to be made in the equipment. The known centralconfigurations and data terminal equipment according to the DAVIC systeminclude the necessary signal-processing components for realizing theknown synchronization of the SL-ESFs and the upstream slots and theediting of the transmission to frames in the downstream direction and tobursts in the upstream direction. Using the mini slots in the upstreamdirection requires only that the operation of these pails isresynchronized so that, on the one hand, the data terminal equipment mayedit the transmission to short mini bursts and, on the other hand, thecentral configuration may separate the successive mini bursts from eachother and handle them as data units related to different data terminalconnections. The resynchronization is most advantageously carried outprogrammably by making the necessary changes in the program processed bythe microprocessor controlling the central configuration, on the onehand, and the data terminal equipment, on the other. Likewise, it ispossible to realize the proposed new mini slot reception indicatormessage and the identification in the data terminal equipmentprogrammatically.

The connection from the cable television further to other datatransmission systems, i.e. so-called trunkline networks may beestablished in may ways. In the DAVIC system it has been proposed that aso-called A4-type interface separates the central configuration fromgeneral ATM networks. Because the arrangement of the invention in caseof call connections transmits similar delay-critical data of 64 kbps asthe known digital telephone systems, it might be advantageous toestablish a direct connection from the cable television system to thegeneral SDH network (Synchronous Digital Hierarchy), SONET network(Synchronous Optical NETwork), or PDH network (Plesiochronous DigitalHierarchy) so that the data stream of 64 kbps related to the callconnection and transmitted by the mini slots would as such be compatiblewith the data transmission format used in the trunkline network. If anATM trunkline network connection in accordance with the DAVICspecifications is used, there are at least two possibilities to make themini slots and the ATM cells of the trunkline network compatible.According to the first principle, the contents of one mini slot are setto correspond to one ATM cell of the trunkline network so that no extradelay is caused in the data transmission, but a large part of themainline network capacity is wasted, as the ATM cell could accommodatefour times as much data as the mini slot. The other alternative is, incase of an upstream connection, to gather the data of four successiveupstream mini slots and use them to fill one ATM cell, which would thenbe transmitted to the trunkline network. Correspondingly, in downstreamdirection, the central configuration unpacks the ATM cell arriving fromthe trunkline network and distributes its contents to four successivedownstream mini slots. In this case, the extra data transmission delayis the same as would be caused by the generation of state-of-the-art ATMcells in a data terminal equipment. However, the echo suppressingrequired by the delay may be carried out in the trunkline network andnot in the data terminal equipment of the user as in thestate-of-the-art system, which is advantageous, as the data terminalequipment need not contain an echo suppressing operation so that itsmanufacturing costs can be kept lower.

The control of the use of mini slots is not limited to the reservationarrangement according to the DAVIC system, but also other, moreefficient reservation arrangements may be used for assigning the slotsfor the data terminal equipment. In addition, what is said about cabletelevision systems above and in appended claims, may be generalized toconcern data transmission systems, the protocols and operationalrelations resembling cable television systems, however so that the datatransmission is carried out by radio waves instead of a cable. Examplesof such systems are known MMDS (Microwave Multipoint DistributionServices) and LMDS (Local Multipoint Distribution Services) systems.

Enclosure 1

Calculation algorithm of M bits according to the state of the art:

    __________________________________________________________________________    if(downstream.sub.-- rate==3.088 Mbit/s) {n = 1;}                             else {n = 0;}                                                                 upstream.sub.-- slot.sub.-- position.sub.-- register==value of M-bits         latched at bit.sub.-- position M11 (M10-M1)                                   if(upstream.sub.-- rate==1.544 Mbit/s) {m = 3;}                               else if(upstream.sub.-- rate==3.088 Mbit/s) {m = 6;}                          else {m = 0.5)                                                                if(bit.sub.-- position==M1 and previous M12==1)                               {upstream.sub.-- slot.sub.-- position.sub.-- counter = upstream.sub.--        slot.sub.-- position.sub.-- register * 3 * m;}                                if(bit.sub.-- position==M5)                                                   if((n==0) or (n==1 and previous M12==0))                                      {upstream.sub.-- slot.sub.-- position.sub.-- counter = upstream.sub.--        slot.sub.-- position.sub.-- counter + m;}                                     if(bit.sub.-- position==M9)                                                   if((n = 0) or (n = 1 and previous M12==1))                                    {upstream.sub.-- slot.sub.-- position.sub.-- counter = upstream.sub.--        slot.sub.-- position.sub.-- counter + m;}                                     if(bit.sub.-- position==M11)                                                  {temp.sub.-- upstream.sub.-- slot.sub.-- position.sub.-- register = (M10,     M9, M8, ...., M1);}                                                           if((bit.sub.-- position==M12) and (M12==1))                                   {upstream.sub.-- slot.sub.-- position.sub.-- register = temp.sub.--           upstream.sub.-- slot.sub.-- position.sub.-- register;}                        __________________________________________________________________________

Calculation algorithm of M bits according to the invention:

    __________________________________________________________________________    if(downstream.sub.-- rate==3.088 Mbit/s) {n = 1;}                             else {n = 0;}                                                                 upstream.sub.-- slot.sub.-- position.sub.-- register = value of M-bits        latched at bit.sub.-- position M11 (M10-M1)                                   if(upstream.sub.-- rate==1.544 Mbit/s) {m = 9;}                               else if(upstream.sub.-- rate==3.088 Mbit/s) {m = 18;}                         else {m = 1.5}                                                                if(bit.sub.-- position==M1 and previous M12==1)                               {upstream.sub.-- slot.sub.-- position.sub.-- counter = upstream.sub.--        slot.sub.-- position.sub.-- register * 3 * m;}                                if(bit.sub.-- position==M5)                                                   if((n==0) or (n==1 and previous M12==0))                                      {upstream.sub.-- slot.sub.-- position.sub.-- counter = upstream.sub.--        slot.sub.-- position.sub.-- counter + m;}                                     if(bit.sub.-- position==M9)                                                   if((n = 0) or (n = 1 and previous M12==1))                                    {upstream.sub.-- slot.sub.-- position.sub.-- counter = upstream.sub.--        slot.sub.-- position.sub.-- counter + m;}                                     if (bit.sub.-- position==M11)                                                 {temp.sub.-- upstream.sub.-- slot.sub.-- position.sub.-- register = (M10,     M9, M8, ...., M1);}                                                           if((bit.sub.-- position==M12) and (M12==1))                                   {upstream.sub.-- slot.sub.-- position.sub.-- register = temp.sub.--           upstream.sub.-- slot.sub.-- position.sub.-- register;}                        __________________________________________________________________________

Enclosure 2

Connect message according to the state of the art in table form:

    __________________________________________________________________________                                    Bit                                                                           Number/                                                                       Descrip-                                      Connect.sub.-- Message (){                                                                             Bits                                                                             Bytes                                                                             tion                                          __________________________________________________________________________    Connection.sub.-- ID     32 4                                                 Session.sub.-- Number    32 4                                                 Resource.sub.-- Number   16 2                                                 Connection.sub.-- Control.sub.-- Field                                                                  1                                                   DS.sub.-- ATM.sub.-- CBD.sub.-- Included                                                                1     7: {N/Y}                                      DS.sub.-- MPEG.sub.-- CBD.sub.-- Included                                                               1     6: {N/Y}                                      US.sub.-- ATM.sub.-- CBD.sub.-- Included                                                                1     5: {N/Y}                                      Upstream.sub.-- Channel.sub.-- Number                                                                   3     4..2                                          Slot.sub.-- List.sub.-- Included                                                                        1     1: {N/Y}                                      Cyclic.sub.-- Assignment  1     0: {N/Y}                                      Frame.sub.-- Length      16 2                                                 Maximum.sub.-- Contention.sub.-- Access.sub.--                                                          8 1                                                 Message.sub.-- Length                                                         Maximum.sub.-- Reservation.sub.-- Access.sub.--                                                         8 1                                                 Message.sub.-- Length                                                         if(Connection.sub.-- Control.sub.-- Field==                                   DS.sub.-- ATM.sub.-- CBD.sub.-- Included){                                    Downstream.sub.-- ATM.sub.-- CBD()                                                                     64 8                                                 if(Connection.sub.-- Control.sub.-- Field==                                   DS.sub.-- MPEG.sub.-- CBD.sub.-- Included){                                   Downstream.sub.-- MPEG.sub.-- CBD()                                                                    48 6                                                 }                                                                             if(Connection.sub.-- Control.sub.-- Field==                                   US.sub.-- ATM.sub.-- CBD.sub.-- Included){                                    Upstream.sub.-- ATM.sub.-- CBD()                                                                       64 8                                                 }                                                                             if(Connection.sub.-- Control.sub.-- Field==                                   Slot.sub.-- List.sub.-- Included){                                            Number.sub.-- Slots.sub.-- Defined                                                                      8 1                                                 for(i=0; i<Number.sub.-- Slots.sub.-- Assigned;i++){                          Slot.sub.-- Number       13 2                                                 }                                                                             }                                                                             if(Connection.sub.-- Control.sub.-- Field==                                   Cyclic.sub.-- Assignment){                                                    Contentionless.sub.-- Start                                                                            16 2                                                 Contentionless.sub.-- Dist                                                                             16 2                                                 Contentionless.sub.-- End                                                                              16 2                                                 }                                                                             }                                                                             __________________________________________________________________________

connect message according to the invention in table form:

    __________________________________________________________________________                                    Bit                                                                           Number/                                                                       Descrip-                                      Connect.sub.-- Message (){                                                                             Bits                                                                             Bytes                                                                             tion                                          __________________________________________________________________________    Connection.sub.-- ID     32  4                                                Session.sub.-- Number    32  4                                                Resource.sub.-- Number   16  2                                                Connection.sub.-- Control.sub.-- Field                                                                  2                                                   reserved                  5     11...15                                       Second.sub.-- Cyclic.sub.-- Assignment.sub.-- Included                                                  1     10                                            DS.sub.-- STM.sub.-- CBD.sub.-- Included                                                                1      9                                            US.sub.-- STM.sub.-- CBD.sub.-- Included                                                                1      8                                            DS.sub.-- ATM.sub.-- CBD.sub.-- Included                                                                1      7: {N/Y}                                     DS.sub.-- MPEG.sub.-- CBD.sub.-- Included                                                               1      6: {N/Y}                                     US.sub.-- ATM.sub.-- CBD.sub.-- Included                                                                1      5: {N/Y}                                     Upstream.sub.-- Channel.sub.-- Number                                                                   3     4..2                                          Slot.sub.-- List.sub.-- Included                                                                        1      1: {N/Y}                                     Cyclic.sub.-- Assignment  1      0: {N/Y}                                     Frame.sub.-- Length      16  2                                                Maximum.sub.-- Contention.sub.-- Access.sub.--                                                          8  1                                                Message.sub.-- Length                                                         Maximum.sub.-- Reservation.sub.-- Access.sub.--                                                         8  1                                                Message.sub.-- Length                                                         if(Connection.sub.-- Control.sub.-- Field==                                   DS.sub.-- STM.sub.-- CBD.sub.-- Included){                                    Upstream.sub.-- STM.sub.-- CBD()                                                                       64  8                                                if(Connection.sub.-- Control.sub.-- Field==                                   US.sub.-- STM.sub.-- CBD.sub.-- Included){                                    Downstream.sub.-- STM.sub.-- CBD()                                                                     64  8                                                }                                                                             if(Connection.sub.-- Control.sub.-- Field==                                   DS.sub.-- ATM.sub.-- CBD.sub.-- Included){                                    Downstream.sub.-- ATM.sub.-- CBD()                                                                     64  8                                                }                                                                             if(Connection.sub.-- Control.sub.-- Field==                                   DS.sub.-- MPEG.sub.-- CBD.sub.-- Included){                                   Downstream.sub.-- MPEG.sub.-- CBD()                                                                    48  6                                                }                                                                             if(Connection.sub.-- Control.sub.-- Field==                                   US.sub.-- ATM.sub.-- CBD.sub.-- Included){                                    Upstream.sub.-- ATM.sub.-- CBD()                                                                       64  8                                                }                                                                             if(Connection.sub.-- Control.sub.-- Field==                                   Slot.sub.-- List.sub.-- Included){                                            Number.sub.-- Slots.sub.-- Defined                                                                      8  1                                                for(i=0; i<Number.sub.-- Slots.sub.-- Assigned;i++){                          Slot.sub.-- Number       13  2                                                }                                                                             }                                                                             if(Connection.sub.-- Control.sub.-- Field==                                   Cyclic.sub.-- Assignment){                                                    Contentionless.sub.-- Start                                                                            16  2                                                Contentionless.sub.-- Dist                                                                             16  2                                                Contentionless.sub.-- End                                                                              16  2                                                }                                                                             if(Connection.sub.-- Control.sub.-- Field==                                   Second.sub.-- Cyclic.sub.-- Assignment){                                      Contentionless.sub.-- Start                                                                            16  2                                                Contentionless.sub.-- Start.sub.-- Second                                                              16  2                                                Contentionless.sub.-- Dist                                                                             16  2                                                Contentionless.sub.-- End                                                                              16  2                                                }                                                                             }                                                                             __________________________________________________________________________

Enclosure 3

Connection descriptor of a downstream STM connection in table form:

    ______________________________________                                                                          Bit                                                                           Number/                                                                       Descrip-                                    Downstream.sub.-- STM.sub.-- CBD(){                                                             Bits    Bytes   tion                                        ______________________________________                                        Downstream.sub.-- Frequency                                                                     32      4                                                   Downstream.sub.-- STM.sub.-- ID                                                                 16      2                                                   Downstream.sub.-- Type                                                                           8      1       {enum}                                      ______________________________________                                    

Connection descriptor of an upstream STM connection in table form:

    ______________________________________                                                                          Bit                                                                           Number/                                                                       Descrip-                                    Upstream.sub.-- STM.sub.-- CBD () {                                                             Bits    Bytes   tion                                        ______________________________________                                        Upstream.sub.-- Frequency                                                                       32      4                                                   Upstream.sub.-- STM.sub.-- ID                                                                   16      2                                                   Upstream.sub.-- Parameters                                                                              1                                                   MAC.sub.-- Flag.sub.-- Set                                                                       5          7..3                                            Upstream.sub.-- Rate                                                                             3          2..0:                                                                         {enum}                                          ______________________________________                                    

Enclosure 4

Mini slot reception indicator message in table form:

    ______________________________________                                                                        Bit                                                                           Number/                                                                       Descrip-                                      Mini.sub.-- Slot.sub.-- Reception.sub.-- Indicator.sub.-- Message()                                    Bytes  tion                                          ______________________________________                                        First.sub.-- Mini.sub.-- Slot                                                                          16     2                                             Number.sub.-- of.sub.-- Mini.sub.-- Slots                                                               8     1                                             for(i=0; i<Number.sub.-- Mini.sub.-- Slots;i++){                              Reservation ID or STM.sub.-- ID                                                                        16     2                                             }                                                                             ______________________________________                                    

What is claimed is:
 1. A method for transmitting digital data in a channel of a cable television system applying time division multiple access in which slots of a defined slot length are assigned for terminal equipment in order to distribute the use of data transmission capacity to the equipment, and the use of slots in the cable TV system are controlled by use indications transmitted downstream, characterized in that the slots are further divided into mini slots, the use of which is controlled by the indications transmitted downstream wherein the mini slots are divided into successive mini slot groups of regular and equal temporal duration, and for ensuring that a specific equipment has a specific regular data transmission capacity, it is assigned at least one mini slot of a specific mini slot group so that the assigned mini slot is repeated cyclically at intervals of a certain number of mini slot groups.
 2. The method of claim 1, characterized in that the length of three mini slots plus a guard byte is the same as the defined slot length.
 3. The method of claim 1, characterized in that the cyclically repeated mini slot assigned for a specific equipment is repeated in every mini slot group and that it is always situated at the same place in relation to the start and end of the mini slot group.
 4. The method of claim 1, characterized in that the cyclically repeated mini slot assigned for a specific equipment is repeated in mini slot groups between which there is a certain number of mini slot groups, in which the corresponding mini slot has not been assigned for the use of the equipment in question.
 5. The method of claim 1, characterized in that, as a method, it is compatible with DAVIC 1.0 and 1.1 specifications.
 6. The method of claim 1, characterized in that the digital data contains delay-critical data.
 7. the method of claim 1, characterized in that, for ensuring that a specific equipment has a certain regular data transmission capacity, it is assigned plural mini slots from a mini slot group so that when the assigned mini slots are repeated cyclically at intervals of a specific number of mini slot groups, they are always situated at the same place in relation to the start and end of the mini slot group.
 8. A cable television system comprising a central configuration and several data terminal equipment in a data transmission connection where data is transmitted by cables, the central configuration comprising means for receiving data from data terminal equipment in accordance with a time division principle in slots, and means for establishing control commands to assign said slots for the use of separate data terminal equipment; in which cable television system, at least one data terminal equipment includes means for receiving control commands from the central configuration, and means for transmitting digital data to the central configuration in burst form in a slot defined by the control commands; characterized in that the central configuration comprises means for resynchronizing the control commands so that they assign mini slots for the use of separate data terminal equipment, which are temporally shorter than the slots, and that the data terminal equipment comprises means for transmitting data as mini bursts the size of a mini slot so that the data terminal equipment is provided to transmit data as bursts or mini bursts according to what kind of control commands it receives from the central configuration, and that the mini slots are divided into successive mini slot groups of regular and equal temporal duration, and for ensuring that a specific equipment has a specific regular data transmission capacity, it is assigned at least one mini slot of a specific mini slot group so that the assigned mini slot is repeated cyclically at intervals of a certain number of mini slot groups.
 9. The method of claim 8, characterized in that the cyclically repeated mini slot assigned for a specific equipment is repeated in every mini slot group and that it is always situated at the same place in relation to the start and end of the mini slot group.
 10. The method of claim 9, characterized in that the cyclically repeated mini slot assigned for a specific equipment is repeated in mini slot groups between which there is a certain number of mini slot groups, in which the corresponding mini slot has not been assigned for the use of the equipment in question.
 11. The method of claim 9, characterized in that, for ensuring that a specific equipment has a certain regular data transmission capacity, it is assigned plural mini slots from a mini slot group so that when the assigned mini slots are repeated cyclically at intervals of a specific number of mini slot groups, they are always situated at the same place in relation to the start and end of the mini slot group.
 12. The method of claim 9, characterized in that the digital data contains delay-critical data.
 13. The method of claim 9, characterized in that, as a method, it is compatible with DAVIC 1.0 and 1.1 specifications. 